1. Field of the Invention
The invention relates to a semiconductor memory device, and more particularly, to an NAND flash memory for programming and erasing.
2. Description of Related Art
In a programming operation of a flash memory, electrons are accumulated at a floating gate to shift a threshold voltage of a memory cell in a positive direction. In an erasing operation, electrons are released from the floating gate to shift the threshold voltage of the memory cell in a negative direction. The programming and erasing operations must be performed by controlling threshold values of the memory cells to fall within distribution ranges of “0” and “1”. Also, whether the programming and erasing operations are qualified are determined through a programming verification and an erasing verification (Patent Document 1).